Re: How to declare a generic formal type "covered" by another?

["J-P. Rosen" <rosen@adalog.fr>]

Le 01/05/2014 09:33, Natasha Kerensikova a écrit :
> The examples I had in mind are:
>   - T is Stream_Element_Array, S is Stream_Element_Array (1 .. 4096)
This is an unconstrained type and a constrained subtype of it.

>   - T is Root_Stream_Type'Class, S is a concrete type derived from it
This is an indefinite type, and a definite type derived from it.

>> 3) There is no way to say that S must be constrained, because being
>> constrained is a matter of subtype, and only types matter for generics.
> 
> Well maybe the problem here is my limited vocabulary. I used the word
> "constrained" there only to mean "Something := new S" or "Variable : S;"
> are both valid, i.e. objects of that type can be created without
> further information.
This is definite vs. indefinite types.

> There must be some way to express that in generics.
An unknown discriminant in a formal, i.e. "(<>)", expresses that the
type may be indefinite.

Now, thinking about it... When you derive a formal from an indefinite
type, it stays indefinite. OTOH, you can derive from a definite type and
give unknown discriminants to make indefinite. Add to that that a type
is considered derived from itself, and the following compiles OK:

procedure Essai is
   generic
      type T is private;
      type S (<>) is new T;
   procedure P;
   procedure P is
      X : T;
   begin
      null;
   end;
   subtype CS is String (1..10);
   procedure Inst is new P (CS, String);
begin
   null;
end Essai;

Close enough to what you wanted?
-- 
J-P. Rosen
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